Squid giant axons injected with arsenazo III as an intracellular Ca indicator will also be treated with TTX and TEA to pharmacologically block Na and K currents and will then be voltage clamped from a variety of resting potentials initially specified by changing the [K] in seawater. The family of curves for internal Ca vs. membrane potential will allow one to characterize the dependence of [Ca] in axoplasm on membrane potential. Other measurements will examine the dependence of Ca efflux on membrane potential. Finally, the factors influencing the inactivation of Na/Ca exchange in squid axons will be examined.